Mainly in refinery and petrochemical industry the need for experienced plumbers and skilled welders is great. The requirements of the tubing and any welded joints within this type of industry are of obvious reasons very high, and to provide high quality weld seams and an efficient workflow of welding operations is currently in normal cases, a small group consisting of one experienced plumber and 2-3 experienced welders working together. A tube fitter rigs tubes and fittings, which are to be welded together, and the tube fitter employs 2-3 welders which welds the rigged tube parts together. The rigging is currently made usually with tube fixtures which are mounted on the tube or fitting by gripping the tube/fitting from the outside in three or more points. In the fixed position then both the rough and fine adjustment can be performed, to center one tube relative to another tube, provided with a tube fixture. The fixture thus grip from the outside and the adjustment is made from the outside, but it is the inside that is most important in terms of centering, because no or minimal interior edge misalignment may occur between the tube parts to obtain a good and safe welding, according to current standards. Therefore the centering is checked visually from the inside, often with help from a flashlight, and the need of adjustment is determined, whereafter fine-tuning is performed with adjustment screws at the tube fixture. This is therefore a method built on that the tube fitter possesses great experience, feel and accuracy. To mention in connection to this is that for example tube bends have slightly different wall thickness—thicker material in the inner radius and thinner material in the outer radius. Also details as T-couplings, but also the tube itself may have different wall thickness. While it is very important for the quality of the weld that it is the inside that has as little edge misalignment as possible, relative to next tube part, which should be welded to the actual tube part, the method of internal visual inspection of the joint together with the external adjustment constitutes a blunt and uncertain method, basically completely built on the experience of the tube fitter. The method is also time-consuming and the tube fitter cannot employ more than 2-3 welders with this method. Another difficulty is that for example connecting tube fittings, as flanges, T-couplings and tube bends, which shall be welded with a certain angle in relation to the first tube part, for the further route of the tubing. With today's equipment it is often hard to arrange the rigging with the exact angle and at the same time a perfect centered joint, also having the correct distance between the tube parts before the forthcoming welding. Thus, it is almost impossible to use internal visual inspection of the joint while it is a number of bends or T-couplings and similar that will be welded together. Similar problems are also at hand for long tubes. But there is an advantage with this kind of external tube fixture by that no interior tools needs to be removed before or after the welding operation.
Tools have been developed in order to take into account that it is the inside of the tube or fitting that is most important to center relative to the connecting tube. In U.S. Pat. No. 3,283,115 an interior clamp element is used for centering the insides such that a welding device can be used and U.S. Pat. No. 4,496,093 is showing a device for centering tube parts to each other with an interior clamp, which clamp arms expands outwardly to center the tube through a number of operations. Thus, both solutions are difficult to apply on longer tube parts or at bends and the like because they are positioned inside the tubes and somehow needs to be taken out from the tube parts. Further, it is very time-consuming to, as for example in U.S. Pat. No. 4,496,093, perform a number of measurements to decide the inner diameter in respective tube part, adjust two of the clamp arms (29) to the exact radius value, and further compensate for eventual differences in the interior diameter with setting screws (37, 38), and after this insert the tool in the joint and expand a third arm (21) for centering of the tool. A simplified and quicker method is needed.